This invention relates to a drilling device provided with a percussion generator, for earth drilling purposes, wherein the percussion generator is arranged to be actuated by a flow of liquid introduced into a borehole through a drill pipe, and wherein an oscillating percussion force exerted on a drilling tool attachable to the percussion generator is produced by means of a spring-loaded thrust valve which is in communication with the drill pipe and which controls the flow of liquid.
Devices of this general type are known for example from U.S. Pat. Nos. 2,388,741 and 3,018,834, and also from British patent specification No. 2776 (A.D. 1913). In these known devices the flow of flushing liquid flows through the thrust valve, which is held open by a spring arrangement with a high velocity and under high pressure, in order to act on a drilling tool which is fixed to a stem or piston of the thrust valve. As soon as the hydrodynamic forces of the flow of liquid against the thrust valve are greater than the opening force of the spring of the thrust valve, the thrust valve closes abruptly, so that, as a consequence of the high velocity and the high pressure of the flow of liquid supplied through the drill pipe, a pressure pulse is produced which acts against the stem or the piston of the thrust valve and is transmitted to the tool as a percussive movement. By virtue of this percussive movement the flow of liquid loses part of its energy, so that the spring of the thrust valve then opens the valve again and the process which produces the stroke can begin anew.
The known devices of this type have various disadvantages however which, until now, have prevented a trouble-free utilization of the known devices. Apart from the fact that in the known devices the flushing liquid which flows in must have a high speed and a high pressure, it has also been necessary that the thrust valve should have a sealing fit, in order to be able to guarantee the impulse pressure necessary for the movement stroke of the tool. Such a sealing fit of the thrust valve is susceptible to damage however since the flow of flushing liquid has a high dirt content. This can only be overcome by filtration, which results in increased expenditure. If the fit of the thrust valve is damaged, then the known devices can only function with reduced efficiency or, in the worst cases, can no longer function.
A further important disadvantage of the known devices is that the renewed opening of the thrust valve is not certain, since, only the movement stroke of the stem or of the piston of the thrust valve has an influence on the reduction of the liquid pressure acting on the thrust valve. With unfavorable speed or pressure conditions of the flow of flushing liquid and/or if there is dirt in the thrust valve which acts in a manner to block the thrust valve from fitting properly, one can have the result that the opening force of the thrust valve is not sufficiently great to open the valve again, so that the device is no longer functional.
Finally, the known devices also have the disadvantage that the various parameters, namely the velocity and the pressure of the flow of flushing liquid, the aperture of the thrust valve, the spring force of the thrust valve, and possibly the force of a restoring-spring of the thrust valve, etc., must be determined accurately in relation to each other in order to be able to maintain the operation of the devices within defined limits.